Discover how to measure battery state-of-charge and what future developments may bring.
The life of a battery cannot be defined by the number of cycles or age alone but by how the battery is used. As the capacity fades, the discharge time gets shorter. The smart battery captures the changes but these vital health statistics often remain hidden from the user. This turns a battery into a “black box,” concealing the performance records and disguising when the battery should be replaced.
One of the main tasks of the smart battery is to establish communication between the battery and the user. A fuel gauge indicating state-of-charge fulfills part of this. When pressing the TEST button on a fully charged SMBus battery, all signal lights illuminate. On a partially discharged battery, half the lights illuminate, and on an empty battery all lights remain dark or a red light appears. Figure 1 shows a fuel gauge of a battery that is 75 percent charged with three lights glowing.
Figure 1: State-of-charge readout of a “smart” battery
Signal lights indicate the battery SoC when pressing the TEST button.
Courtesy of Cadex
While the SoC information displayed on a battery or a display screen is helpful to the user, the readout does not guarantee runtime. The fuel gauge resets to 100 percent on a full recharge regardless of how much capacity the battery can store. A serious breach can occur if an aged battery shows 100 percent SoC while the battery’s ability to hold charge has dropped to 50 percent or less. We ask, “100 percent of what?” If, for example, 100 percent of a good battery results in a four-hour runtime, a battery holding half the capacity would run for only two hours. Many users are not aware that the fuel gauge only shows SoC; capacity, the leading health indicator, remains unknown.
Other than doing a full discharge with a controlled current and measuring time, there is no reliable method to calculate the state-of-health (SoH) of a battery. A full discharge is normally done as part of maintenance and calibration. However, there is a digital way to estimate the capacity of a smart battery.
At time of manufacture, the SMBus battery is programmed with a specified capacity, which is 100 percent by default, and the battery keeps this information as permanent data. With each full charge, the battery resets to the full-charge flag; and during discharge the coulomb counter measures the consumed energy. A perfect battery would deliver 100 percent on a calibrated fuel gauge. As the battery ages and the capacity drops, the delivered energy between charges decreases. The discrepancy between the factory-set 100 percent and the delivered coulombs after a full charge can be determined as the full charge capacity (FCC), reflecting the digital equivalent of a full discharge.
Coulomb counting can also estimate SoH during charging, and this works best with an empty battery. A battery with a 100 percent capacity will receive the full coulomb-count; one with only 50 percent will accept only half the coulomb count before the battery reaches full-charge.
Not knowing the exact SoC at the beginning of the coulomb count on charge will result in inaccuracies. SoC can be estimated by measuring the battery’s open circuit voltage (OCV), but this only gives a rough approximation. Agitation after charge or discharge, temperature and diverse cathode materials affect the OCV readings in Li-ion. (See BU-903: How to Measure State-of-charge.)
Coulomb counting is also done on discharge and this normally begins after a full charge. What remains unknown is the remaining capacity until reaching the end-of-discharge point. Periodic calibrations with a full discharge enable a complete coulomb count that is compared with the factory setting to represent the digital capacity equivalent.
The SoC and capacity information can be shown on a linear display using colored LEDs. The green lights indicate the usable capacity; the empty part of the battery is marked with un-lit LEDs; and the unusable part is shown with red LEDs. Figure 2 illustrates a tri-state fuel gauge. The results can be a shown on a digital display.
Figure 2: Tri-state fuel gauge.The tri-state fuel gauge reads the “learned” battery information on the SMBus and displays it on a multi colored LED bar. The illustration shows a partially discharged battery of 50% SoC with 20% empty and 30% unusable.
Courtesy of Cadex
The tri-state fuel gauge provides state-of-function (SoF), the ultimate in battery diagnostics, but device manufacturers are hesitant to offer this feature to consumers. Batteries age, even during the warranty, and giving too much information could cause concerns and an increase in warranty claims. Device manufacturer are obliged to furnish a warranty replacement if the capacity drops to below 80 percent. Keeping this information hidden is seen as the least disruptive method. SoF may be accessed by a service code.
Cars with electric propulsion systems do not show the remaining charge as with a liquid fuel gauge. Instead, EVs indicate the remaining driving range, hiding the storage capacity. To accommodate capacity fade that would shorten the driving range, EV batteries are oversized and at first do not use the full charge and discharge range. As the battery ages, the usable range gradually expands. Shorted driving ranges will only become visible once all reserve capacity is consumed. (See BU-1003: Electric Vehicle.)
Last updated 2015-11-21
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